Wireless handheld or portable devices typically operate one or more cellular communication standards and/or wireless connectivity standards, each standard being allocated in one or more frequency bands, and said frequency bands being contained within one or more regions of the electromagnetic spectrum.
For that purpose, a space within the wireless handheld or portable device is dedicated to the integration of an antenna system. However, the antenna system is usually expected to be small in order to occupy as little space as possible within the device, which then allows for smaller devices, or for the addition of more specific equipment and functionality into the device. At the same time, it is sometimes required for the antenna system to be flat since this allows for slim devices or in particular, for devices which have two parts that can be shifted or twisted against each other.
Many of the demands for wireless handheld or portable devices also translate to specific demands for the antenna systems thereof.
A wireless handheld device must include an antenna system capable of operating in multiple frequency regions with a proper radioelectric performance (such as for example in terms of input impedance level, impedance bandwidth, gain, efficiency, or radiation pattern). Moreover, the integration of the antenna system within the wireless handheld device must be correct to ensure that the wireless device itself attains a proper radioelectric performance (such as for example in terms of radiated power, received power, or sensitivity).
For a proper wireless connection, high gain and efficiency are further required. Other more common design demands for antenna systems are the voltage standing wave ratio (VSWR) and the impedance which is supposed to be about 50 ohms. Also, a substantially omnidirectional radiation pattern is desired since a mobile user needs to operate the wireless handheld device regardless of the particular location and direction of the nearest base transceiver station.
Other demands for antenna systems for wireless handheld or portable devices are low cost and a low specific absorption rate (SAR).
Furthermore, an antenna system has to be integrated into a device or in other words a wireless handheld or portable device has to be constructed such that an appropriate antenna system may be integrated therein which puts constraints by consideration of the mechanical fit, the electrical fit and the assembly fit.
Of further importance, usually, is the robustness of the antenna system which means that the antenna system does not change its properties upon smaller shocks to the device.
Usually, antenna systems with a substantially planar conducting radiating element placed at some distance over a ground plane layer are known as microstrip or patch antennas. Usually such microstrip and patch antennas include at least a feeding connection and a grounding connection, forming a so-called Planar Inverted F Antenna (PIFA). It is well known that the performance of such antennas is limited, in terms of impedance bandwidth, efficiency and related parameters (gain, VSWR and so on) by the spacing between said radiating element and the ground plane layer: the shorter the distance between both, the smaller the impedance bandwidth and efficiency.
Typically, a multiband antenna system includes a radiating element whose geometry is able to support different radiation modes so that said antenna system can operate with a determined radioelectric performance in multiple frequency regions of the electromagnetic spectrum. Such a multiband behavior is achieved by appropriately shaping the geometry of the radiating element, creating several radiating geometric elements, such as arms, polygons, or straight or curved line segments, and/or introducing slots, apertures or openings within the radiating element, to provide different paths to the electric currents flowing on the conductive parts of the radiating element and/or to the equivalent magnetic currents on slots, apertures or openings within said radiating element, exciting different radiation modes for the multiple frequency regions of operation.
In such multiband antenna systems, the different radiation modes supported by the radiating element are usually associated to different geometrical portions of said radiating element. Said portions need to share a same volume dedicated to the integration of the radiating element inside the wireless handheld device, and in fact, in most cases, they have to compete for space within said volume.
Therefore, these antenna systems typically exhibit a limited radioelectric performance in the frequency regions of operation.
Moreover, the presence of multiple arms, slots, apertures and/or openings within the radiating element of these antenna systems makes them much more sensitive to external effects (such as for instance the presence of plastic or dielectric covers that surround the wireless device), to components of the wireless device (such as for instance, but not limited to, a speaker, a microphone, a connector, a display, a shield can, a vibrating module, a battery, or an electronic module or subsystem) placed either in the vicinity of, or even underneath, the radiating element, and/or to the presence of the user of the wireless device.
A multiband antenna system is sensitive to any of the above mentioned aspects because they may alter the electromagnetic coupling between the different geometrical portions of the radiating element, which usually translates into detuning effects, degradation of the radioelectric performance of the antenna system and/or the radioelectric performance wireless device, and/or greater interaction with the user (such as an increased level of SAR).
On the other hand, antenna systems having a radiating element with a geometrically simple shape (such as for example those composed by one polygon) provide a single path for the currents, exciting just one radiation mode.
In these antenna systems, the radiating element comprises a single geometrical portion that uses all the available volume within the wireless handheld device dedicated to the antenna system. Therefore these antenna systems may provide an improved radioelectric performance and be less sensitive to external effects and/or to the presence of components of the device in a neighborhood of the radiating element, and possibly be less affected by the presence of the user.
However, such antenna systems typically provide only one frequency region of operation, in which the antenna system exhibits a good radioelectric performance. This aspect constitutes a major limitation to the use of said antenna systems in wireless handheld or portable devices, because these devices usually operate one or more communication standards requiring multiple frequency regions of the electromagnetic spectrum.
Moreover, different communication standards being operated by a wireless handheld or portable device are usually allocated in distantly spaced frequency regions. For example, frequencies within a frequency region being two times higher than frequencies in another frequency region are typical in wireless devices operating GSM 900 (880-960 MHz) and GSM 1800 (1710-1880 MHz), or operating IEEE 802.11b/g (2.4-2.5 GHz) and IEEE 802.11a (5.15-5.875 GHz).
Therefore, an antenna system having a radiating element with a geometrically simple shape and featuring good radioelectric performance over a broad or wide frequency region (i.e., a broadband or wideband solution) able to encompass the plurality of frequency regions of operation of a wireless handheld or portable device is impractical.
Some attempts have been made to obtain an antenna system capable of operating in two frequency regions, in which said antenna system comprises a radiating element of a geometrically simple shape.
For example, U.S. Pat. No. 6,674,411 discloses a planar inverted-L antenna (i.e., a patch antenna) having a radiating element composed by a rectangular plate. Said radiating element is placed above, and substantially parallel to, a ground plane. The antenna is connected to a matching network that provides a match in two frequency regions of the electromagnetic spectrum.
As a further example, US 2005/0225484 describes a planar inverted-F antenna (PIFA) having a radiating element composed by a rectangular plate. Said radiating element is placed above, and substantially parallel to, a ground plane. The antenna is connected to a matching circuit that performs adjustment of the reflection characteristics of the antenna in two frequency regions.
Moreover, the PIFA comprises a feeding connection and a grounding connection. In order to achieve the desired operation in multiple frequency regions, the feeding connection and the grounding connection must be disposed at a distance not smaller than one sixth of the circumference length of the radiating element.
These prior-art antenna systems are however very sensitive to the height of the radiating element with respect to the ground plane. For the typical 5-15% impedance bandwidths of cellular/mobile standards (GSM, UMTS, PCS, WCDMA), the minimum distance is about 2% of the longest operating wavelength (typically 7-9 mm). As the height of the radiating element is reduced, the impedance bandwidth and the efficiency of the antenna system decrease. Consequently, the capability of the antenna system to operate in more than one frequency region, or with a broadband behavior, is severely diminished.
Such a severe limitation makes these antenna systems not suitable for many wireless handheld or portable devices, in particular for those devices in which the volume dedicated to the integration of the antenna system has a limited height, or in those thin, slim devices in which the overall height of the device itself has to be small.